In the 1950s and early 1960s, when I was in graduate school, realism was the reigning doctrine in the field of "international relations." The standard reference was not to justice but to interest. Moral argument was against the rules of the discipline as it was commonly practiced, although a few writers defended interest as the new morality. There were many political scientists in those years who preened themselves as modern Machiavellis and dreamed of whispering in the ear of the prince; and a certain number of them, enough to stimulate the ambition of the others, actually got to whisper. They practiced being cool and tough-minded; they taught the princes, who did not always need to be taught, how to get results through the calculated application of force. Results were understood in terms of "the national interest," which was the objectively determined sum of power and wealth here and now plus the probability of future power and wealth. More of both was almost always taken to be better; only a few writers argued for the acceptance of prudential limits; moral limits were, as I remember those years, never discussed. Just war theory was relegated to religion departments, theological seminaries, and a few Catholic universities. And even in those places, isolated as they were from the political world, the theory was pressed toward realist positions; perhaps for the sake of self-preservation, its advocates surrendered something of its critical edge.

Vietnam changed all this, although it took a while for the change to register at the theoretical level. What happened first occurred in the realm of practice. The war became a subject of political debate; it was widely opposed, mostly by people on the left. These were people heavily influenced by Marxism; they also spoke a language of interest; they shared with the princes and professors of American politics a disdain for moralizing. And yet the experience of the war pressed them toward moral argument. Of course, the war in their eyes was radically imprudent; it could not be won; its costs, even if Americans thought only of themselves, were much too high; it was an imperialist adventure unwise even for the imperialists; it set the United States against the cause of national liberation, which would alienate it from the Third World (and significant parts of the First). But these claims failed utterly to express the feelings of most of the war's opponents, feelings that had to do with the systematic exposure of Vietnamese civilians to the violence of American war-making. Almost against its will, the left fell into morality. All of us in the antiwar camp suddenly began talking the language of just war--though we did not know that that was what we were doing.

It may seem odd to recall the '60s in this way, since today the left seems all too quick to make moral arguments, even absolutist moral arguments. But this description of the contemporary left seems to me mistaken. A certain kind of politicized, instrumental, and highly selective moralizing is indeed increasingly common among leftist writers, but this is not serious moral argument. It is not what we learned, or ought to have learned, from the Vietnam years. What happened then was that people on the left, and many others too, looked for a common moral language. And what was most available was the language of just war. We were, all of us, a bit rusty, unaccustomed to speaking in public about morality. The realist ascendancy had robbed us of the very words that we needed, which we slowly reclaimed: aggression, intervention, just cause, self-defense, noncombatant immunity, proportionality, prisoners of war, civilians, double effect, terrorism, war crimes. And we came to understand that these words had meanings. Of course, they could be used instrumentally; that is always true of political and moral terms. But if we attended to their meanings, we found ourselves involved in a discussion that had its own structure. Like characters in a novel, concepts in a theory shape the narrative or the argument in which they figure.
The triumph of just war theory (Michael Walzer, Winter 2002, Social Research)

A funny thing happened to Michael Walzer on his way to opposing the
Vietnam War. Recognizing the hollowness of the arguments being made by
the post-modern Left he sought serious moral ground and found it in
traditional Just War Theory. But morality is a tricky thing, for just
when we think it's conveyed a cloak for the selfish actions we wish to
take, or not take as the case may be, we discover that it has
simultaneously imposed certain responsibilities upon us, often unwanted
ones, always selfless ones. Thus, while Mr. Walzer grabbed onto Just War
Theory as a way to oppose a given war, he discovered that: "[J]ust war
theory, even when it demands a strong critique of particular acts of
war, is the doctrine of people who expect to use power and exercise
force." The theory, obviously, assumes that war can be just. Indeed,
Mr. Walzer has developed the notion that war is justified in instances
of "supreme emergency," occasions where "our deepest values and our
collective survival are in imminent danger." Moreover, while recognizes
the importance of self-determination as a principle for relations among
nations, he adds a liberal democratic component to the standard.
Accepting the biblical injunction "Do not stand idly by the blood of
they neighbor" he accepts the justness of humanitarian intervention in
cases where a nation's people are "victims of tyranny, ideological zeal,
[and/or] ethnic hatred" and who "urgently need help from outside."

Though Mr. Walzer remains very much a man of the Left in general, this
recognition that humanitarian intervention may be morally justified/obligated against regimes that do not meet standards of liberal democratic legitimacy has put him so much at odds with the rest of the Left that they accuse him of being a crypto-conservative
and he has been forced to ask whether there can even be a Decent Left, a
question which he largely
answers in the negative. The essays in this book are drawn from the
past twenty-five years, with an emphasis on our recent interventions in
Iraq (twice), Kosovo, Afghanistan, Haiti, etc. and our tragic
non-intervention in Rwanda. Because he straddles the line between Left and Right he's unlikely to satisfy anyone completely. But you can't help but admire the utter gravity with which he reckons with every issue and, even where you disagree, the depth of his arguments forces you to reckon with them yourself. The Decent Left may be a deuced small collection of folk, but counting Mr. Walzer among their number is no small boast.